Tunable, capacitive components are normally provided by varactor diodes. In varactor diodes, the junction capacitance is dependent on an applied control voltage.
Varactor diodes or tunable, capacitive components are used in “voltage controlled oscillators” (VCOs), for example in large-scale productions. Voltage controlled oscillators in a resonant circuit normally comprise a fixed-value inductance in addition to a tunable capacitance and are therefore called LC oscillators. Varying the capacitance value of the tunable capacitance sets the oscillation frequency of the oscillator. Such voltage controlled oscillators are required in transmission and reception appliances, for example, in a mobile radio.
FIG. 1 shows a basic circuit arrangement for a voltage controlled LC oscillator having two inductances 1, two tunable capacitances 2 and two cross-coupled NMOS transistors 3 based on the prior art with symmetrical design. The oscillation frequency Fosc of the LC VCO shown in FIG. 1 is determined, in a good approximation, by the resonant frequency of the LC circuit, which is dependent on the product of effective inductance and effective capacitance. The frequency Fosc of the LC VCO can therefore be controlled by the voltage controlled capacitances 2, which can be set using the voltage Vtune. The LC oscillator comprising the actual LC resonant circuit 1, 2 and the damping reduction amplifier 3 coupled thereto is fed by a reference current source 4.
In FIG. 1, the controlling voltage Vtune is applied to the controlled capacitances 2 at their connecting node, which forms the circuit input 5. A pair of circuit nodes 6, 7 forms the output of the circuit, so that the controlling voltage Vtune applied externally to reference potential connection 8 respectively does not impress the controlling voltage at the (capacitance) control inputs of the varactors 2 directly. The voltages at the nodes 6, 7 oscillate with a 180-degree phase shift at the frequency Fosc and the amplitude Uosc around a mean voltage value which, when measured with respect to ground, is dependent on the current Iref and the layout of the transistors 3. The current Iref in the reference current source 4 shown in FIG. 1 is never totally independent of the supply voltage in practice, which means that disturbances in the supply voltage are projected into the reference current Iref. In addition, even a reference current source is never totally noise- and disturbance-free itself.
Disturbances in the reference current Iref bring about co-directional fluctuations in potential at the outputs A and B of the LC VCO and are thus projected into the mean voltage value. Since the mean capacitance value of the varactors 2 is determined directly by the tuning voltage applied across the varactors 2, disturbances in the reference current Iref also alter the frequency Fosc or the phase of the LC VCO at the same time. Random disturbances in the frequency or phase of an oscillator oscillation are normally observed in the form of phase noise in the course of measurements.
For low LC VCO phase noise, it is therefore important, inter alia, for the change in the capacitance 2 with the controlling voltage not to be unnecessarily high. It is therefore desirable if the voltage range over which the capacitance 2 can be controlled is as large as possible, and at the same time the voltage dependency of the capacitance 2 is linear over the entire control voltage range.
So that amplitude noise is not transformed into phase noise, it is also advantageous if the controlled capacitance value is not a function of the voltage amplitude which is present across the capacitance 2.
So that the setting range for the capacitance 2 is available completely under all operating conditions, it is also important for the voltage dependency of the capacitance 2 to be independent of temperature across the entire control voltage range.
FIG. 2a shows a voltage controlled capacitance 2 based on the prior art with two normally off NMOS transistors 9 whose four source/drain connections are connected to the tuning input 5. The output node pair 6, 7 is connected to a respective gate connection on the transistors 9.
The circuit arrangement shown in FIG. 2a may be used unchanged for the LC VCO shown in FIG. 1. Co-directional fluctuations in potential at the connections 6, 7 in comparison with connection 5 respectively bring about—within the control range of the capacitances per unit length between the gate and the transistor channel—a co-directional change in the capacitances per unit length in the NMOS transistors 9 between the gates and the interconnected drain and source connections.
For the arrangement shown in FIG. 2a, the change in the capacitance or in the capacitances per unit length with the control voltage, which has an effective drop across the transistors 9 operating as varactor, is comparatively very large and is linear only in a very small range, which is smaller than 50 mV. If the amplitude Uosc at the outputs 6, 7 of the LC VCO is very large, the situation improves somewhat.
FIG. 2b shows the electrical equivalent circuit diagram of the tunable capacitance from FIG. 2a. 
The document P. Andreani et al. “A 2.2 GHz CMOS VCO with Inductive Degeneration Noise Suppression”, IEEE 2001 Custom Integrated Circuit Conference, pp. 197□200, IEEE/CICC 2001, ISBN 0-7803-6591-7 specifies a controllable capacitance in a VCO which is constructed using MOS transistors.
The document C. Samori et al. “A −94 dBc/Hz@100 kHz fully integrated 5-GHz CMOS VCO with 18% tuning range for Bluetooth Applications”, IEEE 2001 Custom Integrated Circuit Conference, pp. 201–204, IEEE. CICC 2001, ISBN 0-7803-6591-7 specifies an LC VCO in which the tunable components used are PMOS varactors.
The common drawback of the known tunable capacitive components suitable for use in VCOs is that they do not have a large linear tuning range and/or relatively high series resistances. High series resistances result in poor quality, while too small a tuning range for a varactor or for a tunable capacitance entails the drawback, generally when used in a VCO, of just a narrow frequency band in which the VCO can be tuned. Finally, a low level of linearity results in circuit properties being dependent on the present operating point of the capacitive component, which in turn has a disadvantageous effect on the implementation of control loops etc.